Report CPU Time alongside Wall Time

We'd like to be able to quantify CPU time consumed while executing
the candidate and control blocks during science experiments.

Changes include:
- Added `cpu_time` attribute to `Scientist::Observation`.
- Refactored initialization to capture both start/end times for wall
time and CPU time.
- Updated tests to include CPU-intensive work so we can verify we're
recording CPU time correctly.

This allows us to track cpu time more precisely when making improvements.

Author: elenatanasoiu

lib/scientist/observation.rb

@@ -20,19 +20,29 @@ class Scientist::Observation
   # The Float seconds elapsed.
   attr_reader :duration
 
+  # The Float CPU time elapsed, in seconds
+  attr_reader :cpu_time
+
   def initialize(name, experiment, fabricated_duration: nil, &block)
     @name       = name
     @experiment = experiment
 
-    starting = Process.clock_gettime(Process::CLOCK_MONOTONIC, :float_second) unless fabricated_duration
+    start_wall_time, start_cpu_time = capture_times unless fabricated_duration
+
     begin
       @value = block.call
     rescue *RESCUES => e
       @exception = e
     end
 
-    @duration = fabricated_duration ||
-      Process.clock_gettime(Process::CLOCK_MONOTONIC, :float_second) - starting
+    if fabricated_duration
+      @duration = fabricated_duration["duration"]
+      @cpu_time = fabricated_duration["cpu_time"]
+    else
+      end_wall_time, end_cpu_time = capture_times
+      @duration = end_wall_time - start_wall_time
+      @cpu_time = end_cpu_time - start_cpu_time
+    end
 
     freeze
   end
@@ -89,4 +99,13 @@ def hash
   def raised?
     !exception.nil?
   end
+
+  private
+
+  def capture_times
+    [
+      Process.clock_gettime(Process::CLOCK_MONOTONIC, :float_second),
+      Process.clock_gettime(Process::CLOCK_PROCESS_CPUTIME_ID, :float_second)
+    ]
+  end
 end

test/scientist/experiment_test.rb

@@ -674,27 +674,47 @@ def @ex.enabled?
     it "allows a user to provide fabricated durations for testing purposes" do
       @ex.use { true }
       @ex.try { true }
-      @ex.fabricate_durations_for_testing_purposes( "control" => 0.5, "candidate" => 1.0 )
-
+      @ex.fabricate_durations_for_testing_purposes({
+        "control" => { "duration" => 0.5, "cpu_time" => 0.4 },
+        "candidate" => { "duration" => 1.0, "cpu_time" => 0.9 }
+      })
       @ex.run
 
       cont = @ex.published_result.control
       cand = @ex.published_result.candidates.first
+
+      # Wall Time
       assert_in_delta 0.5, cont.duration, 0.01
       assert_in_delta 1.0, cand.duration, 0.01
+
+      # CPU Time
+      assert_equal 0.4, cont.cpu_time
+      assert_equal 0.9, cand.cpu_time
     end
 
     it "returns actual durations if fabricated ones are omitted for some blocks" do
       @ex.use { true }
-      @ex.try { sleep 0.1; true }
-      @ex.fabricate_durations_for_testing_purposes( "control" => 0.5 )
-
+      @ex.try do
+        start_time = Time.now
+        while Time.now - start_time < 0.1
+          # Perform some CPU-intensive work
+          (1..1000).each { |i| i * i }
+        end
+        true
+      end
+      @ex.fabricate_durations_for_testing_purposes({ "control" => { "duration" => 0.5, "cpu_time" => 0.4 }})
       @ex.run
 
       cont = @ex.published_result.control
       cand = @ex.published_result.candidates.first
+
+      # Fabricated durations
       assert_in_delta 0.5, cont.duration, 0.01
+      assert_in_delta 0.4, cont.cpu_time, 0.01
+
+      # Measured durations
       assert_in_delta 0.1, cand.duration, 0.01
+      assert_in_delta 0.1, cand.cpu_time, 0.01
     end
   end
 end

test/scientist/observation_test.rb

@@ -6,13 +6,18 @@
 
   it "observes and records the execution of a block" do
     ob = Scientist::Observation.new("test", @experiment) do
-      sleep 0.1
+      start_time = Time.now
+      while Time.now - start_time < 0.1
+        # Perform some CPU-intensive work
+        (1..1000).each { |i| i * i }
+      end
       "ret"
     end
 
     assert_equal "ret", ob.value
     refute ob.raised?
     assert_in_delta 0.1, ob.duration, 0.01
+    assert_in_delta 0.1, ob.cpu_time, 0.01
   end
 
   it "stashes exceptions" do